ImageMagick Is On Fire — CVE-2016–3714

TL;DR

There are multiple vulnerabilities in ImageMagick, a package commonly used by web services to process images. One of the vulnerabilities can lead to remote code execution (RCE) if you process user submitted images. The exploit for this vulnerability is being used in the wild.

A number of image processing plugins depend on the ImageMagick library, including, but not limited to, PHP’s imagick, Ruby’s rmagick and paperclip, and nodejs’s imagemagick.

If you use ImageMagick or an affected library, we recommend you mitigate the known vulnerabilities by doing at least one of these two things (but preferably both!):

Verify that all image files begin with the expected "magic bytes" corresponding to the image file types you support before sending them to ImageMagick for processing. (see FAQ for more info)

Use a policy file to disable the vulnerable ImageMagick coders. The global policy for ImageMagick is usually found in “/etc/ImageMagick”. The below policy.xml example will disable the coders EPHEMERAL, URL, MVG, and MSL.

@lcamtuf With Advice On Better Mitigations

@lcamtuf has a post that talks about mitigations you should consider beyond the basic blacklisting recommended in the initial disclosure. You should read it.

Note: Contrary to what is stated in the post, we have recommended sandboxing in our FAQ from the beginning.

What's with the stupid (logo|website|twitter account)?

It would have been fantastic to eschew this ridiculousness, because we all make fun of branded vulnerabilities too, but this was not the right time to make that stand.

Initially, we disclosed this vulnerability via a blog post on Medium. We even joked in the post that the vulnerability didn't have a cool name or logo. The blog was read hundreds of times, but as the hours passed, we became worried that not enough people were aware of the vulnerability. Every script kiddie would have it in their hands soon, but a majority of people had no idea this vulnerability existed.

So we created a website, a twitter account, and used a logo that someone created as a joke the day before.

What happened?

We had thousands of hits in the first 15 minutes. We were at the top of hacker news, which a lot of people see. We were getting the word out on something tragickally simple to exploit. We'd do it again.

Detailed Vulnerability Information

Nikolay Ermishkin from the Mail.Ru Security Team discovered several vulnerabilities in ImageMagick. We've reported these issues to developers of ImageMagick and they made a fix for RCE in sources and released new version (6.9.3-9 released 2016-04-30 changelog), but this fix seems to be incomplete. We are still working with developers.

ImageMagick allows to process files with external libraries. This feature is called 'delegate'. It is implemented as a system() with command string ('command') from the config file delegates.xml with actual value for different params (input/output filenames etc). Due to insufficient %M param filtering it is possible to conduct shell command injection. One of the default delegate's command is used to handle https requests:

"wget" -q -O "%o" "https:%M"

where %M is the actual link from the input. It is possible to pass the value like

The most dangerous part is ImageMagick supports several formats like
svg, mvg (thanks to Stewie for his research of
this file format and idea of the local file read vulnerability in
ImageMagick, see below), maybe some others - which allow to include
external files from any supported protocol including delegates. As a
result, any service, which uses ImageMagick to process user supplied
images and uses default delegates.xml / policy.xml, may be vulnerable to
this issue.

ImageMagick tries to guess the type of the file by it's content, so
exploitation doesn't depend on the file extension. You can rename
exploit.mvg to exploit.jpg or exploit.png to bypass file type checks. In
addition, ImageMagick's tool identify is also vulnerable, so it can't
be used as a protection to filter file by it's content and creates
additional attack vectors (e.g. via less exploit.jpg', because
identify is invoked via lesspipe.sh).

Ubuntu 14.04 and OS X, latest system packages (ImageMagick 6.9.3-7 Q16
x86_64 2016-04-27 and ImageMagick 6.8.6-10 2016-04-29 Q16) and latest
sources from 6 and 7 branches all are vulnerable. Ghostscript and wget
(or curl) should be installed on the system for successful PoC
execution. For svg PoC ImageMagick's svg parser should be used, not rsvg.

All other issues also rely on dangerous ImageMagick feature of external
files inclusion from any supported protocol in formats like svg and mvg.

4. CVE-2016-3716 - File moving

It is possible to move image files to file with any extension in any
folder by using ImageMagick's 'msl' pseudo protocol. msl.txt and
image.gif should exist in known location - /tmp/ for PoC (in real life
it may be web service written in PHP, which allows to upload raw txt
files and process images with ImageMagick):

FAQs

Who found this bug?

Stewie found the initial bug, and Nikolay Ermishkin from the Mail.Ru Security Team found additional issues, including the RCE.

Will you share the exploit with me?

No. We would like to give people a chance to patch before it is more widely available. The exploit is trivial, so we expect it to be available within hours of this post. Updates and PoC will eventually be available here.

What are "magic bytes"?

The first few bytes of a file can often used to identify the type of file. Some examples are GIF images, which start with the hex bytes "47 49 46 38", and JPEG images, which start with "FF D8". This list on Wikipedia has the magic bytes for most common file types.

Why are you disclosing a vulnerability like this?

We have collectively determined that these vulnerabilities are available to individuals other than the person(s) who discovered them. An unknowable number of people having access to these vulnerabilities makes this a critical issue for everyone using this software. ImageMagick also disclosed this on their forum a few hours ago.

How well-tested are these mitigations?

They are effective against all of the exploit samples we’ve seen, but we cannot guarantee they will eliminate all vectors of attack.

Are there other ways to mitigate?

Sandboxing ImageMagick is worth investigating, but we are not providing specific instructions for doing this.

Why is this post so short?

We did not find this vulnerability ourselves. We understand the mechanisms involved, but credit for finding this vulnerability should go to the researcher(s).